This invention relates to new anisometric iron oxide black pigments of great purity and color intensity, a process for the production of such iron oxide black pigments and the use thereof for color substrates.
Iron oxide black pigments are mixed oxides of divalent and trivalent iron corresponding to the composition of magnetite, Fe.sub.3 O.sub.4. They may be obtained by the oxidation of metallic or divalent iron or by the reduction of trivalent iron.
The production of magnetites by the reduction of iron(III) compounds is mainly used in the field of magnetic pigments, in which needle-shaped magnetites, in particular, are used either directly as magnetic pigment or as intermediate for the production of ferrimagnetic .gamma.-Fe.sub.2 O.sub.3 (Ullmanns Encylopadie der techischen Chemie, 4th Edition, Volume 18, page 643, Verlag Chemie, Weinheim, 1979). Magnetites prepared by this process have neither a pure color tone nor exceptional color intensity and have therefore not become established as color pigments.
The iron oxide black pigments obtained on a technical scale by the aniline process of oxidation of metallic iron shown an increasingly strong brown tinge at higher color intensities. This is due to an increased proportion of fine particles in the primary grain distribution.
The isometric iron oxide black pigments obtained in the neutral to slightly basic region from commericial iron salt solutions by precipitation and oxidation have the disadvantage that they readily incorporate in the lattice structure thereof the metal impurities present in the solutions. In particular, the manganese impurities frequently present in iron salt solutions are completely incorporated, and these also produce a brown tinge in black pigment.